Antimony oxide fine particles are used as flame retardants for plastics, fabrics, fibers and the like, coating agents for plastics and glasses, and materials of transparent films having antistatic properties, electromagnetic wave screening function or the like, and they are also known to have electrical conductivity. As processes for preparing an antimony oxide sol containing such antimony oxide fine particles, various processes are already known.
For example, in Japanese Patent Publication No. 11848/1982, there is disclosed a process for preparing a colloidal sol of Sb2O3 having particle diameters of 2 to 100 nm, comprising allowing Sb2O3 to react with KOH and H2O2 in a molar ratio of about 1:2.1:2 (Sb2O3:KOH:H2O2) to form potassium antimonate and then performing deionization. It is also known that, in the formation of a colloidal sol of antimony oxide by the reaction of antimony trioxide with hydrogen peroxide, if an inorganic alkali substance, such as Li, K, Na, Mg, Ca, Ba or sodium phosphate, is added to the reaction system in an amount of 1.5 to 30% by mol based on Sb2O3, the rate of the reaction of Sb2O3 with H2O2 is accelerated and colloidal antimony oxide having extremely small particle diameter is obtained. In Japanese Patent Laid-Open Publication No. 137828/1985, there is described a process for preparing chemically stable colloidal antimony oxide having extremely small particle diameter, said process being characterized in that the molar ratio between Sb2O3 and H2O2 is set to 1:2.25-1.8 and an inorganic alkali substance is added to the reaction system in the formation of a colloidal sol of antimony oxide by the reaction of Sb2O3 with H2O2.
In Japanese Patent Laid-Open Publication No. 41536/1985, it is disclosed that a stable antimony pentaoxide sol of high concentration and low viscosity is obtained by a process comprising allowing alkali antimonate to react with a monovalent or divalent inorganic acid in a stoichiometric amount of 0.7 to 5 times to form an antimony pentaoxide gel, then separating the gel, washing the gel with water and peptizing the gel by an organic base such as amine.
In Japanese Patent Laid-Open Publication No. 227918/1986, there is disclosed that, in a process for preparing an antimony pentaoxide gel comprising peptizing an antimony pentaoxide sol obtained by the reaction of alkali antimonate with a monovalent or divalent inorganic acid, if phosphoric acid is added in the reaction stage and/or the peptization stage so that the P2O5/Sb2O3 wt % should become 0.2 to 5%, an antimony pentaoxide sol having excellent stability is obtained in the formation of an organosol by solvent replacement with an organic solvent.
In Japanese Patent Laid-Open Publication No. 227919/1986, there is disclosed a process for preparing an antimony pentaoxide sol, in which an antimony pentaoxide sol wherein surfaces of colloidal particles having properties that they are not aggregated even if an organic solvent is added are coated with a trivalent and/or tetravalent metal is prepared by mixing an antimony pentaoxide sol with an aqueous solution of at least one basic salt of a trivalent metal and/or a tetravalent metal in a given mixing ratio.
In Japanese Patent Laid-Open Publication 180717/1990, the present applicant has disclosed that an antimony oxide sol containing fine particles and having a homogeneous particle diameter distribution is obtained by setting a molar ratio between antimony trioxide, an alkali substance and hydrogen peroxide to 1:2.0-2.5:0.8-1.5 and adding hydrogen peroxide to the system containing antimony trioxide and an alkali substance at a rate of 0.2 mol/hr based on 1 mol of the antimony trioxide in the preparation of an antimony sol by the reaction of antimony trioxide with an alkali substance and hydrogen peroxide.
The antimony oxide fine particles obtained by the conventional processes, however, are dispersed in a monodisperse state, and therefore, when they are used for a transparent film requiring antistatic properties, the resulting film shows insufficient antistatic properties depending upon the purpose, and adhesion of dirt or dust cannot be prevented in some cases. On this account, increasing the amount of the antimony oxide fine particles added or increasing the film thickness has been made, but in such a case, there is a problem of lowering of film strength and transparency or deterioration of economical efficiency.
It is also known that a hard coating film is formed on a surface of a substrate such as glass, plastic sheet or plastic lens in order to enhance scratch resistance of the substrate surface. To the hard coating film such an organic resin film or an inorganic, film, resin particles or inorganic particles such as silica are added to further enhance the scratch resistance.
Moreover, it is known that an anti-reflection film is formed on a surface of a substrate such as glass, plastic sheet or plastic lens to prevent reflection of the substrate surface. For example, it is known that a film of a low-refractive index material, such as a fluororesin or magnesium fluoride, is formed on a surface of a glass substrate or a plastic substrate by means of coating, deposition or CVD method, or an anti-reflection film is formed by coating a substrate surface with a coating liquid containing low-refractive index fine particles such as silica fine particles. For example, Japanese Patent Laid-open Publication No. 133105/1995 discloses that an anti-reflection substrate is prepared by the use of a sol wherein composite oxide colloidal particles consisting of silica and another inorganic oxide are dispersed. Further, formation of a conductive film containing metal fine particles or conductive oxide fine particles is carried out in order to impart antistatic properties and electromagnetic wave screening properties to the substrate.
Also when an anti-reflection film and/or a conductive film is provided as described above, a hard coating film is formed between the substrate and the anti-reflection film and/or the conductive film in order to enhance the scratch resistance.
In case of a conventional hard coating film, however, adhesion of the film to the substrate and scratch resistance of the film itself become insufficient especially when the substrate is a resin substrate.
Moreover, also when an anti-reflection film and/or a conductive film is provided on the conventional hard coating film, marring takes place after the hard coating film is formed or adhesion of dust due to static electricity takes place. Consequently, transparency or haze of the finally produced substrate with a conductive film is deteriorated, resulting in a problem of lowering of product yield.
In recent years, further, as portable telephones, PDA, notebook type personal computers and liquid crystal television sets, those of small and lightweight type have been used, and therefore, resin substrates have been used as the substrates. For example, acrylic resin substrate, polycarbonate substrate, triacetyl cellulose (TAC) resin substrate and the like have been used. In case of a conventional hard coating film, however, there is a large difference in the refractive index between the conventional hard coating film and these substrates, and interference fringe takes place when light is reflected, resulting in a problem of occurrence of display unevenness such as flickering, glaring and color shading. In addition, there is also a problem of insufficient adhesion of the film to these substrates.
In the light of the above problems, the present inventors have earnestly studied, and as a result, they have found that all of the above problems can be solved by using, instead of monodisperse particles, fine particle groups in each of which inorganic oxide fine particles having particle diameters of a specific range are connected in the form of a chain.
The present inventors have found that when the inorganic oxide fine particle groups are antimony oxide fine particle groups, the resulting transparent film exhibits excellent transparency and excellent antistatic properties, and even when they are other inorganic oxide fine particle groups, a film (hard coating film) having excellent adhesion to a substrate and excellent scratch resistance can be obtained by adding the inorganic oxide fine particle groups to the film, particularly to the hard coating film.